could you use roofing membrane under the C.I. to prevent the condensation dripping down . To be used in external garage.

Essentially yes, you can use a membrane under the steel sheets but its important to fit the membrane to solid boarding otherwise it will sag between the batons.
Alternatively, you can purchase Anti-con sheeting which removes the need for a membrane, but we would advise speaking to a member of our team who could advise further.